The 11th International Conference on the Principles and Practice of Constraint Programming (CP 2005) was held in Sitges (Barcelona), Spain, October 1-5, 2005. Information about the conference can be found on the web at http: //www.iiia.csic.es/cp2005/.Informationaboutpastconferencesinthe series can be found athttp: //www.cs.ualberta.ca/ ai/cp/. The CP conference series is the premier international conference on c- straint programming and is held annually. The conference is concerned with all aspects of computing with constraints, including: algorithms, applications, environments, languages, models and systems. This year, we received 164 submissions. All of the submitted papers received atleastthreereviews, andthepapersandtheirreviewswerethenextensivelyd- cussed during an online Program Committee meeting. As a result, the Program Committee chose 48 (29.3%) papers to be published in full in the proceedings and a further 22 (13.4%)papers to be published as short papers.The full papers werepresentedattheconferencein twoparalleltracksandtheshortpaperswere presented as posters during a lively evening session. Two papers were selected by a subcommittee of the ProgramCommittee-consisting of Chris Beck, Gilles Pesant, and myself-to receive best paper awards. The conference program also includedexcellentinvitedtalksbyH´ ectorGe?ner, IanHorrocks, FrancescaRossi, and Peter J. Stuckey. As a permanent record, the proceedings contain four-page extended abstracts of the invited talks.